A boat’s bonding system is the best protection against electrolysis.
The zincs on your buddy’s boat are lasting only a few months before they need to be replaced, and he’s sure the old boat in the next slip is leaking stray electrical current. Or you take your boat to the yard for some routine maintenance and notice the antifouling paint had released, or “cooked,” around the metal through-hulls and where the trim tabs attach to the hull. You immediately diagnose the problem and declare the marina must have stray current and declare it to be “hot.”
Don’t bet on it. Most stray current problems are sourced on the boat on which they appear. Otherwise, everyone in the marina would have the same problem. But how do you know for sure what the source is? There are a couple tests that can be performed to determine the source of the problem.
AC Stray Current Test
Stray current is the most difficult corrosion problem of all to identify and correct, because the source could be anywhere among the mass of wiring in your boat. The first thing to do is meter out the dock circuit to make sure it isn’t feedback from the dock grounding system. The test should be conducted by the marina with the shore power on and being drawn (e.g., 120v equipment or an appliance running). Our marina checks every shore-power box annually to ensure there’s no stray current.
AC current corrosion occurs much less frequently, mainly due to the fact the high voltage is dangerous, and it’s treated with more respect. Moreover, if there is a 120v leak, chances are it’s going to be found rather quickly — most boat owners don’t appreciate occasional shocks. The number of AC ground faults is few and far between.
DC Stray Current Test
DC current leaks are the most common form of stray current problems. Any boat that utilizes high-quality underwater metals and has a good bonding system can tolerate a fairly high amount of stray current, because the low-voltage current quickly dissipates throughout the system. The small amount that finds its way to underwater metals is usually taken care of by the zincs, or dissipated by a large amount of metal.
To check for stray DC current, a corrosion survey using a silver chloride electrode is conducted, which will determine if there is stray current and, sometimes, the source.
The best protection from stray current is a well-maintained bonding system, but due to its out-of-sight status, it tends to get overlooked when it comes to maintenance.
The purpose of a bonding system is to equalize the electric potential of dissimilar underwater metals by tying them all together with wire. The benefits of a bonding system are wide ranging but little perceived. One is that it serves to dissipate stray current leaks. Twelve volts of current focused on a small piece of metal will result in rapid destruction. But the same 12 volts spread over a much larger surface cause less damage in proportion to the size of the water-exposed surface of metal. Bonding systems can reduce the corrosion potential of metals inside and on the bottom of the boat. A boat that has all the hardware bonded will suffer much less corrosion.
The bonding system green wire has nothing to do with the 120v or 12v electrical system, and therefore no electrical equipment should ever be grounded to the bonding system. Those green wires only serve the purpose of linking together, or bonding, all metal pieces on the boat. Unfortunately, some DIY skippers don’t understand that and use it to ground equipment.
Actually, there are four separate ground systems: DC ground, AC ground, AC grounding (or bond) and the boat’s bonding system. You can add lightning and HF radio grounds.
On most pleasureboats, the green bonding cable is fine-strand tinned copper between 4 and 10 mm. A problem many boat owners encounter is the bonding system is connected in a daisy chain; therefore, there is no redundancy if there is a wire breakage or high resistance. A bonding system should be designed using quality bus bars and tinned copper fine-strand wire, and all fittings should be taken back to bus bars.
Bonding systems are not maintenance free. They utilize wire and ordinary crimped ring terminals. The wire connections corrode, and since electrical current doesn’t flow very well through corroded metal, the connections need to be reestablished periodically, which is done by cutting off the old terminal and connection and installing a new one.
When a fitting is suffering corrosion and it’s bonded into the protection system, your sacrificial anode will work harder to keep it protected, and eventually the anode will wear out quickly, which can have further consequences. A new zinc anode can disappear in two or three months due to failure within the bonding system.
What does bottom paint have to do with electrical systems? With copper-based paints, a lot. Look at your metal through-hulls the next time you take your boat to the yard. If you see large ugly burn patterns around all your metal through-hulls — haloing — you have a stray current problem. A copper-based bottom paint reacts severely to stray current and serves as a great indicator. It’s easier to see the haloing with black paint, as the halo is usually green due to the copper content of the antifouling paint.